Biosensors using an enzyme that specifically reacts with a particular substrate are being actively developed in various industrial fields. As for a glucose sensor, which is one of the biosensors, in particular, measurement methods and devices utilizing such methods are being actively developed mainly in medical fields.
The glucose sensor has a history of about 40 years since Clark and Lyons first reported about a biosensor comprising glucose oxidase and an oxygen electrode in combination in 1962 (L. c. Clark, J. and Lyonas, C. “Electrode systems for continuous monitoring in cardiovascular surgery.” Ann. n. y. Acad. Sci., 105: 20-45).
Thus, the adoption of glucose oxidase as an enzyme of the glucose sensor has a long history. This is because glucose oxidase shows high substrate specificity for glucose and superior thermal stability, this enzyme can further be produced in a large scale, and its production cost is lower than those of other enzymes.
The high substrate specificity means that this enzyme does not react with a saccharide other than glucose, and this leads to an advantage that accurate measurement can be achieved without error in measurement values.
Further, the superior thermal stability means that problems concerning denaturation of the enzyme and inactivation of its enzymatic activity due to heat can be prevented, and this leads to an advantage that accurate measurement can be performed over a long period of time.
However, although glucose oxidase has high substrate specificity and superior thermal stability and can be produced at a low cost, it has a problem that the enzyme is affected by dissolved oxygen as described below and this affects measurement results.
Meanwhile, in addition to glucose oxidase, a glucose sensor utilizing glucose dehydrogenase has also been developed. This enzyme is also found in microorganisms.
For example, there are known glucose dehydrogenase derived from Bacillus bacteria (EC 1.1.1.47) and glucose dehydrogenase derived from Cryptococcus bacteria (EC 1.1.1.119).
The former glucose dehydrogenase (EC 1.1.1.47) is an enzyme that catalyzes a reaction of β-D-glucose+NAD(P)+→D-δ-gluconolactone+NAD(P)H+H+, and the latter glucose dehydrogenase (EC1.1.1.119) is an enzyme that catalyzes a reaction of D-glucose+NADP+→D-δ-gluconolactone+NADPH+H+. The aforementioned glucose dehydrogenases derived from microorganisms are already marketed.
These glucose dehydrogenases have an advantage that they are not affected by oxygen dissolved in a measurement sample. This leads to an advantage that accurate measurement can be achieved without causing errors in measurement results even when the measurement is performed in an environment in which the oxygen partial pressure is low, or a high-concentration sample requiring a large amount of oxygen is used for the measurement.
However, although glucose dehydrogenase is not affected by dissolved oxygen, it has problems of poor thermal stability and substrate specificity poorer than that of glucose oxidase.
Therefore, an enzyme that overcomes disadvantages of both of glucose oxidase and glucose dehydrogenase has been desired.
The inventors of the present invention reported results of their studies about glucose dehydrogenase using samples collected from soil near hot springs in Sode K., Tsugawa W., Yamazaki T., Watanabe M., Ogasawara N., and Tanaka M., Enzyme Microb. Technol., 19, 82-85 (1996); Yamazaki T., Tsugawa W. and Sode K., Appli. Biochemi. and Biotec., 77-79/0325 (1999); and Yamazaki T., Tsugawa W. and Sode K., Biotec. Lett., 21, 199-202 (1999).
However, a bacterial strain having the ability to produce the enzyme had not been identified at the stage of these studies.